The present invention relates to a laser beam system for surveying with a laser incorporated in it, and in particular, to a laser system for surveying, taking influence of heat from the laser into account.
As the laser system for surveying with a laser incorporated in it, various types of laser systems are known such as a rotating laser used in theodolite or interior finish work of buildings, a pipe laser used in piping work, a laser range finder used in measurement of distance, leveling instrument, etc. He-Ne laser and LD (laser diode) are used as the laser light source.
In general, an optical system for leveling or an optical system for range finding in such laser systems for surveying are arranged in a body tube (lens barrel), which is rotatably supported in vertical and horizontal directions in a casing. The laser light source is fixed on the body tube and is integrated with the optical system.
In case He-Ne laser is used as the laser light source, He-Ne tube is large in shape and cannot be designed in compact form. Further, when He-Ne tube is used, a high voltage is required for laser oscillation, and a power source of 100 V is generally used. For this reason, a power consumption is high and a heating value is also high. Accordingly, the power source cannot be incorporated in the system, and it is difficult to design a compact, portable and small-size system.
When the optical system is heated by the heat from the laser light source, detection accuracy of various types of instruments such as a tilt sensor decreases. Or, when a body tube, mechanisms supporting the body tube, etc. are heated, displacement occurs due to thermal expansion, and mechanical accuracy is decreased. For this reason, it is necessary to provide a cooling mechanism and a heat radiating mechanism, and this leads to more complicated and large-scale system.
LD (laser diode) as described above is small in size and low in power consumption, and it is easily incorporated in the body tube, and small-sized portable system can be designed. FIG. 14 schematically shows a pipe laser, which is an example of a laser system for surveying using LD.
An optical system 1 is installed in a body tube 2, which is rotatably supported in vertical and horizontal directions in a casing 4, and a laser light source 3 is fixed on the base of the body tube 2. Laser beam 6 emitted from LD is in an spread angle is large, a collimator lens 5 is arranged to turn the laser beam to parallel beams. Normally, a compensating lens is mounted to turn the elliptical shape to a shape closer to circular.
Compared with the a laser beam from He-Ne tube, the laser beams 6 from LD are not uniform parallel laser beams with a distinct contour, but the beams are spread, and even when the shape of the beams is compensated, the beams are difficult to recognize visually. To improve visual recognizability, an output can be increased, whereas there is legal restriction on intensity of laser beam from the viewpoint of protection of the workers. Therefore, to improve visual recognizability with the same output, it is necessary to select a wavelength, which is easily recognizable. Green color is easily recognized, but semiconductor element to emit high output a green laser beam is not manufactured by mass production. In this respect, a resonator is provided at the laser light source from near infrared LD and a frequency of the laser beam is increased to convert to green color.
As the laser source to emit a green laser beam, LD pumped solid laser is known, which combines oscillator of external or internal resonance type SHG (second harmonic generation) by near infrared semiconductor light emitting element. FIG. 15 is a schematical drawing of a laser source 3, which is LD pumped solid-state laser using such internal resonance type SHG system.
In FIG. 15, reference numeral 8 represents a light emitting unit, and 9 represents an optical resonator. The light emitting unit 8 comprises an LD light emitter 10 and a condenser lens 11. Further, the optical resonator 9 comprises a laser crystal (Nd:YVO.sub.4) plate 13 having a first dielectric reflection film 12 on it, a nonlinear optical medium (KTP) 14, and an output mirror 16 having a second dielectric reflection film 15 on it. In the optical resonator 9, a laser beam is pumped, resonated, amplified and outputted. It is described in more detail as follows:
The laser light source 3 is to generate a laser beam, and the LD light emitter 10, i.e. semiconductor laser, is used. This LD light emitter 10 has a function as a excitation light generator to generate fundamental wave. The laser light source 3 is not limited to semiconductor laser, but any light source means may be adopted if it can generate laser beam.
Laser crystal plate 13 is to amplify light. As the laser crystal plate 13, YAG (yttrium aluminum garnet) doped with Nd.sup.3+ ion is used. YAG has oscillation lines at 946 nm, 1064 nm, 1319 nm, etc.
The laser crystal plate 13 is not limited to YAG, but (Nd:YVO.sub.4) having an oscillation line at 1064 nm or (Ti:Sapphire) having an oscillation line at 700 to 900 nm may be used.
On the side of the laser crystal plate 13 closer to the LD light emitter 10, a first dielectric reflection film 12 is formed. This first dielectric reflection film 12 is highly transmitting to the LD light emitter 10 and is highly reflective to an oscillation wavelength of the laser crystal plate 13. It is also highly reflective to SHG (second harmonic generation).
The output mirror 16 is placed at face-to-face position to the laser crystal plate 13. The side of the output mirror 16 closer to the laser crystal plate 13 is fabricated in form of a concave spherical mirror having an adequate radius, and a second dielectric reflection film 15 is formed on it. The second dielectric reflection film 15 is highly reflective to an oscillation wavelength of the laser crystal plate 13 and is highly transmitting to SHG (second harmonic generation).
As described above, when the first dielectric reflection film 12 of the laser crystal plate 13 is combined with the output mirror 16 and the light beam coming from the LD light emitter 10 is pumped to the laser crystal plate 13 via the condenser lens 11, a light is reciprocally irradiated between the first dielectric reflection mirror 12 of the laser crystal plate 13 and the output mirror 16, and the light can be trapped for long time. Thus, the light can be resonated and amplified.
In the optical resonator comprising the first dielectric reflection film 12 of the laser crystal plate 13 and the output mirror 16, the nonlinear optical medium 14 is placed.
Here, brief description will be given on a nonlinear optical effect.
When an electric field is applied on a substance, electric polarization occurs. When the electric field is small, the polarization is proportional to the electric field. However, in case of a strong coherent light such as a laser beam, proportional relationship between the electric field and the polarization breaks down, and nonlinear polarization component proportional to the square or the cube of the electric field becomes prominent.
Therefore, in the nonlinear optical medium 14, a component proportional to square of a lightwave electric field is contained in the polarization generated by the light wave. By nonlinear polarization, a coupling occurs between lightwaves having different frequencies, and a higher harmonic wave to double the light frequency is generated. This secondary higher harmonic wave generation (SHG) is called second harmonic generation.
In the laser light source 3 as described above, the nonlinear optical medium 14 is placed in the optical resonator, which comprises the laser crystal plate 13 and the output mirror 16, and it is called internal type SHG. Because a converted output is proportional to the square of fundamental wave opto-electric power, a high light intensity in the optical resonator can be directly utilized.
As the nonlinear optical medium 14, KTP (KTiOPO.sub.4 ; titanyl potassium phosphate), BBO (.beta.-BaB.sub.2 O.sub.4 ; .beta.-lithium borate), LBO (LiB.sub.3 O.sub.5 ; lithium triborate), etc. are used, Primarily, it is converted from 1064 nm to 532 nm.
KNbO.sub.3 (potassium niobate) and the like are also adopted, and it is primarily converted from 946 nm to 473 nm.
In general, to stabilize an output wavelength of the laser light source, an output laser beam is monitored and it is fed back to the laser source. FIG. 16 is a block diagram of feedback of an oscillator of internal resonance type SHG mode. A light source unit 60 comprises a laser light source 3, a half-mirror 61 and a condenser lens 17.
A part of the laser beam outputted from the laser light source 3 is split by the half-mirror 61 as a monitor beam. After passing through the half-mirror 61, the laser beam advances toward the condenser lens 17. The monitor beam is received by a monitor photodetector 62 and a photodetection circuit 63 and is converted to an electric signal. A photodetection signal from the photodetection circuit 63 is inputted to a control unit 66, which outputs a control signal corresponding to the photodetection signal to an LD drive unit 67. The LD drive unit 67 controls light emission of the LD light emitter 10 based on the control signal.
In the LD excitation solid laser combined with the oscillator of internal resonance type SHG mode as described above, efficiency is lower than a single semiconductor laser element, and heat is generated more. Accordingly, when the LD pumped solid-state laser is mounted on a casing where an optical system such as telescope is accommodated, accuracy is decreased due to thermal expansion or thermal displacement as in the case of laser light source of He-Ne tube system.